We have used this excellent custom background since 2024 intro

 by Nate Finch

how would you change the Origin Feat to include Expanded Rules like Arcana Unleashed?

You'd have to recreate the background with an expanded feat list. (As far as I know, there's no way to base one's own homebrew on somebody else's.)

(Or leave the feat unchosen and pick it directly on the character sheet.)

Yeah, jl8e's right, homebrew backgrounds can't inherit from someone else's. Easiest fix is to clone Nate's background and just swap in the wider feat list.

One thing that's saved me time: if the builder lets you filter Origin Feat choice by category instead of listing feats one by one, use that. Then you don't have to go back and update the background every time a new origin feat gets released. If the tool doesn't support category filtering though, manual clone with the explicit list is the way to go.

Quote from sriwahyuni >>

One thing that's saved me time: if the builder lets you filter Origin Feat choice by category instead of listing feats one by one, use that. Then you don't have to go back and update the background every time a new origin feat gets released. If the tool doesn't support category filtering though, manual clone with the explicit list is the way to go.

It doesn't, unfortunately. The concept of feats being in different categories that had mechanical meanings is a new thing for 5.5e, and the homebrew system was designed long before 5.5e existed and still largely stuck in the 5e era.